The present invention relates to a heat roll fixing unit for fusing an unfixed toner image and fixing it on a recording sheet by biasing and pressing a backup roll to cause the sheet against the heat roller to be heated and holding the recording sheet bearing the toner therebetween.
Copying machines, electrophotographic printers (laser printers and the like) are well known as imaging apparatus utilizing the so-called electrophotographic system for forming a latent image by exposing the circumferential surface of a photoconductive drum to light, making toner stick to the latent image to develop an image, and transferring the toner image to a recording sheet and then fixing the toner image thereto by means of fixing unit.
There are many methods of fixing toner on recording sheet in a electrophotographic system, including those by means of heat-fusion, the use of a solvent medium, pressure and the like. However, the heat-fusion method is generally employed as safest and most reliable. Among the heat-fusion methods, ther is the frequently employed so-called heat roll method of fixing the heat-fusing toner with heat applied by a heat roller while abutting and pressing a recording sheet bearing an unfixed toner image against the heated roller (heat roller).
The heat roll fixing method comprises the steps of providing a pair of fixing rollers which consists of a heat roller heated to high temperatures by a heat source such as a built-in halogen lamps or the like and a backup roller made of elastic material such as silicon rubber and press-biased thereagainst a predetermined pressure. A recording sheet bearing unfixed toner is held between the pair of rollers, and the toner is fixed by means of heat generated from the heat roller. In general, the heat roller is driven to rotate the backup roller and the recording sheet is fed by the rotation of the pair of fixing rollers.
The rotational drive of the heat roller is normally derived from a gear fixedly mated with the end thereof. Due to the fact that the heat roller is in the form of cylinder whose hollow inside has to contain a heating means (heating source), the gear is fixed to the heat roller as shown in FIG. 1.
More specifically, a slit 1A is formed on one side edge face of a heat roller 1, whereas a protrusion 2A having a predetermined mating tolerance to the slit 1A is formed in the inner periphery of a gear 2 to be mated with the heat roller 1. The protrusion 2A is fitted in the slit 1A to fixedly mate the gear 2 with the heat roller 1. In other words, by forming the slit 1A, equivalent to a key groove, at one end of the heat roller 1, and the protrusion 2A, equivalent to a key, is formed on the inner periphery of the gear 2, whereby the protrusion 2A is fixedly fitted in the slit 1A. The heat roller 1 is generally made of aluminum alloy and the gear 2 of resin such as PPS (polyphenylene sulfide) offering excellent heat resistance and mechanical properties (abrasion resistance, etc.) and suitable for integrally molding the protrusion 2A. Numeral 5 in FIG. 1 denotes a drive gear for transmitting rotational drive force by engaging with the gear 2.
With the heat roller 1 made of aluminum alloy and the gear 2 fixedly fitted thereto and made of resin, however, the heat roller 1 is heated at high temperatures (180.degree. C.-200.degree. C.). Because of the difference in thermal expansion coefficient therebetween (that of resin being greater), the dimensional error of the mating portion tends to increase and the mating portion also tends to become softened. With the drive force applied in such a condition, serious problems arise including deformation of the gear 2, as shown by an imaginary line "S" of FIG. 1B, into a shape close to an ellipse with a line segment "X--X" as its long axis passing through the center of rotation apart from the protrusion 2A (the section blocking relative rotation of the gear with respect to the heat roller 1) by a predetermined angle and, as a result, a periodic change in rotation speed or, in the worst case, malfunction of the motor used as the drive source caused by increased drive resistance.
In the case where a continuous form is used as a recording sheet to be carried by a fixing unit, line space dislocation in printing, irregularity in the half-tone section when an image is formed and other disadvantages occur as the speed of conveying the continuous form changes.